1. Technical Field of the Invention
The present invention generally relates to Claus sulfur recovery plants and to processes for recovering sulfur from H2S-containing gas streams. More particularly, the invention relates to Claus processes and apparatus in which the combustion stage is replaced by a catalytic partial oxidation stage. The invention relates still more particularly to compact systems that require less plot space than conventional modified Claus plants operating at the same sulfur production capacity.
2. Description of the Related Art
Sulfur-recovery plants, also called Claus plants, are well known for removing hydrogen sulfide gas (H2S) resulting from petroleum refining processes and other industrial processes by converting the H2S to elemental sulfur. A conventional modified Claus process includes two primary stages: a thermal or combustion stage and a catalytic stage. In the thermal stage, which is carried out in a furnace, the H2S gas is contacted with a stoichiometric amount of air or a mixture of oxygen and air in a flame so that about one third (⅓) of the H2S is combusted according to the chemical equation:H2S+ 3/2O2→SO2+H2O  (1).Equation 1 is highly exothermic and not limited by equilibrium. Still in the reaction furnace, a portion of the uncombusted H2S (i.e., about ⅔ of the initial amount in the feed) reacts with some of the sulfur dioxide (SO2) product to form elemental sulfur (S0) and water vapor according to the chemical equation:2H2S+SO23/xS0x+2H2O  (2)(x=2, 6, or 8 depending on the temperature and pressure). Chemical Equation 2, which is sometimes referred to as the “Claus reaction,” is endothermic, and the extent of conversion of the H2S and SO2 to elemental sulfur is limited by the chemical equilibrium of the reaction. In the thermal stage a total of about 55 to 70% of the H2S in the original feed is converted to elemental sulfur. To improve the yield, the reacted gases are cooled in a fire tube boiler after emerging from the reaction furnace and elemental sulfur is condensed from the gas stream and removed in molten form, whereupon the gases enter a catalytic stage, which is carried out in a series of catalytic reactors.
In the catalytic stage, the gases are reheated and then passed over a high surface area catalyst bed that promotes the Claus reaction and further converts the process stream to elemental sulfur according to the Claus reaction. Typical Claus catalysts are alumina and titania. Because of the reversible chemical equilibrium of the Claus reaction (Equation 2), the formed products can react according to the reverse Claus reaction (Equation 3)3/xS0x+2H2O2H2S+SO2  (3)with the effect of reducing the efficiency of the Claus plant. The reverse Claus reaction becomes more pronounced as reactor temperature increases. By removing formed elemental sulfur from the process gas exiting the thermal stage, the forward Claus reaction is made more favorable, in accordance with Le Chatlier's Principle. In the catalytic stage, the remaining H2S is reacted with the SO2 (at lower temperatures, i.e., about 200-350° C.) over a catalyst to make more sulfur. Additional catalytic reactors are necessary to remove sequential increments of sulfur. Factors like concentration, flow rate and reaction temperature influence the reaction. From one to four sequential stages of reheating, catalytic reacting and condensing are usually employed industrially. In a typical modified Claus plant in which two or three catalytic reactors are employed, about 90 to 98% of the H2S originally fed to the plant is recovered as elemental sulfur. When endeavoring to go beyond the 90-98% level of sulfur removal, Claus reactors become ineffective; therefore, other measures to remove sulfur from the effluent must be taken.
A conventional modified Claus process is typically employed for processing large quantities of gases containing a high concentration (i.e., >40 vol. %) H2S in Claus plants, producing more than 7,000 tons of sulfur per year. The modified Claus plants in use today are normally operated at less than 2 atmospheres pressure. Because of this low pressure, the pipes and vessels have very large diameters for the flow compared to most refinery or gas plant processes. The low pressure operation forces the equipment to be designed for low pressure drop to have adequate capacity. Further complicating the matter, as ever stricter requirements of regulatory agencies mandate greater efficiency from sulfur recovery plants, it is now common for Claus plants to include tail gas treatment unit. A drawback of adding such equipment to improve sulfur recovery is the further decrease in plant capacity due to increased resistance to flow from additional friction. In order to reduce the frictional pressure loss, the flow of gas through the unit is usually slowed further by modifying the plant design so that the cross-sectional area of the equipment is even larger than before, and the Claus catalyst beds are made shorter, and plant capacity is diminished. Another disadvantage is that the larger sized equipment is more expensive to build and causes the sulfur plant to take up even more plot space. In order to be in compliance with applicable environmental regulations today, a typical modified Claus plant and the necessary tail gas treatment units, constitute a great deal of equipment and occupy a large space. Consequently, there is a need in the art for high capacity sulfur recovery plants and processes that can meet or exceed current sulfur emission standards, and yet are simple in design and more compact than conventional sulfur recovery plants and processes. A way to avoid some of the high capital costs and operating costs of Claus operations in use today would be welcomed by the industry.